Array versus List T When to use which

Question

MyClass   array  List lt MyClass gt  list    What are the scenarios when one is preferable over the other  And why

User · Answer

Rather than going through a comparison of the features of each data type, I think the most pragmatic answer is "the differences probably aren't that important for what you need to accomplish, especially since they both implement IEnumerable, so follow popular convention and use a List until you have a reason not to, at which point you probably will have your reason for using an array over a List."

Most of the time in managed code you're going to want to favor collections being as easy to work with as possible over worrying about micro-optimizations.

User · Answer

They may be unpopular  but I am a fan of Arrays in game projects  - Iteration speed can be important in some cases  foreach on an Array has significantly less overhead if you are not doing much per element - Adding and removing is not that hard with helper functions  - Its slower  but in cases where you only build it once it may not matter - In most cases  less extra memory is wasted  only really significant with Arrays of structs  - Slightly less garbage and pointers and pointer chasing  That being said  I use List far more often than Arrays in practice  but they each have their place   It would be nice if List where a built in type so that they could optimize out the wrapper and enumeration overhead

User · Answer

If I know exactly how many elements I m going to need  say I need 5 elements and only ever 5 elements then I use an array  Otherwise I just use a List lt T

User · Answer

It is rare  in reality  that you would want to use an array  Definitely use a List lt T gt  any time you want to add remove data  since resizing arrays is expensive  If you know the data is fixed length  and you want to micro-optimise for some very specific reason  after benchmarking   then an array may be useful   List lt T gt  offers a lot more functionality than an array  although LINQ evens it up a bit   and is almost always the right choice  Except for params arguments  of course   -p  As a counter - List lt T gt  is one-dimensional  where-as you have have rectangular  etc  arrays like int    or string     - but there are other ways of modelling such data  if you need  in an object model   See also    How When to abandon the use of Arrays in c  net  Arrays  What s the point    That said  I make a lot of use of arrays in my protobuf-net project  entirely for performance    it does a lot of bit-shifting  so a byte   is pretty much essential for encoding  I use a local rolling byte   buffer which I fill before sending down to the underlying stream  and v v    quicker than BufferedStream etc  it internally uses an array-based model of objects  Foo   rather than List lt Foo gt    since the size is fixed once built  and needs to be very fast    But this is definitely an exception  for general line-of-business processing  a List lt T gt  wins every time

User · Answer

Since no one mention  In C   an array is a list  MyClass   and List lt MyClass gt  both implement IList lt MyClass gt    e g  void Foo IList lt int gt  foo  can be called like Foo new     1  2  3    or Foo new List lt int gt    1  2  3       So  if you are writing a method that accepts a List lt MyClass gt  as an argument  but uses only subset of features  you may want to declare as IList lt MyClass gt  instead for callers  convenience   Details    Why array implements IList  How do arrays in C  partially implement IList lt T gt

User · Answer

They may be unpopular  but I am a fan of Arrays in game projects  - Iteration speed can be important in some cases  foreach on an Array has significantly less overhead if you are not doing much per element - Adding and removing is not that hard with helper functions  - Its slower  but in cases where you only build it once it may not matter - In most cases  less extra memory is wasted  only really significant with Arrays of structs  - Slightly less garbage and pointers and pointer chasing  That being said  I use List far more often than Arrays in practice  but they each have their place   It would be nice if List where a built in type so that they could optimize out the wrapper and enumeration overhead

User · Answer

Another situation not yet mentioned is when one will have a large number of items  each of which consists of a fixed bunch of related-but-independent variables stuck together  e g  the coordinates of a point  or the vertices of a 3d triangle    An array of exposed-field structures will allow the its elements to be efficiently modified  in place --something which is not possible with any other collection type   Because an array of structures holds its elements consecutively in RAM  sequential accesses to array elements can be very fast   In situations where code will need to make many sequential passes through an array  an array of structures may outperform an array or other collection of class object references by a factor of 2 1  further  the ability to update elements in place may allow an array of structures to outperform any other kind of collection of structures   Although arrays are not resizable  it is not difficult to have code store an array reference along with the number of elements that are in use  and replace the array with a larger one as required   Alternatively  one could easily write code for a type which behaved much like a List lt T gt  but exposed its backing store  thus allowing one to say either MyPoints Add nextPoint   or MyPoints Items 23  X    5    Note that the latter would not necessarily throw an exception if code tried to access beyond the end of the list  but usage would otherwise be conceptually quite similar to List lt T gt

User · Answer

It completely depends on the contexts in which the data structure is needed  For example  if you are creating items to be used by other functions or services using List is the perfect way to accomplish it   Now if you have a list of items and you just want to display them  say on a web page array is the container you need to use

User · Answer

If I know exactly how many elements I m going to need  say I need 5 elements and only ever 5 elements then I use an array  Otherwise I just use a List lt T

User · Answer

Unless you are really concerned with performance  and by that I mean   Why are you using  Net instead of C     you should stick with List lt    It s easier to maintain and does all the dirty work of resizing an array behind the scenes for you   If necessary  List lt   is pretty smart about choosing array sizes so it doesn t need to usually

User · Answer

Really just answering to add a link which I m surprised hasn t been mentioned yet  Eric s Lippert s blog entry on  Arrays considered somewhat harmful    You can judge from the title that it s suggesting using collections wherever practical - but as Marc rightly points out  there are plenty of places where an array really is the only practical solution

User · Answer

Populating a list is easier than an array  For arrays  you need to know the exact length of data  but for lists  data size can be any  And  you can convert a list into an array   List lt URLDTO gt  urls   new List lt URLDTO gt      urls Add new URLDTO         key    wiki       url    https              urls Add new URLDTO         key    url       url    http              urls Add new URLDTO         key    dir       url    https                 convert a list into an array  URLDTO   return urls ToArray

User · Answer

Really just answering to add a link which I m surprised hasn t been mentioned yet  Eric s Lippert s blog entry on  Arrays considered somewhat harmful    You can judge from the title that it s suggesting using collections wherever practical - but as Marc rightly points out  there are plenty of places where an array really is the only practical solution

User · Answer

If I know exactly how many elements I m going to need  say I need 5 elements and only ever 5 elements then I use an array  Otherwise I just use a List lt T

User · Answer

Lists in  NET are wrappers over arrays  and use an array internally   The time complexity of operations on lists is the same as would be with arrays  however there is a little more overhead with all the added functionality   ease of use of lists  such as automatic resizing and the methods that come with the list class    Pretty much  I would recommend using lists in all cases unless there is a compelling reason not to do so  such as if you need to write extremely optimized code  or are working with other code that is built around arrays

User · Answer

Lists in  NET are wrappers over arrays  and use an array internally   The time complexity of operations on lists is the same as would be with arrays  however there is a little more overhead with all the added functionality   ease of use of lists  such as automatic resizing and the methods that come with the list class    Pretty much  I would recommend using lists in all cases unless there is a compelling reason not to do so  such as if you need to write extremely optimized code  or are working with other code that is built around arrays

User · Answer

Unless you are really concerned with performance  and by that I mean   Why are you using  Net instead of C     you should stick with List lt    It s easier to maintain and does all the dirty work of resizing an array behind the scenes for you   If necessary  List lt   is pretty smart about choosing array sizes so it doesn t need to usually

User · Answer

Arrays Vs  Lists is a classic maintainability vs  performance problem  The rule of thumb that nearly all developers follow is that you should shoot for both  but when they come in to conflict  choose maintainability over performance  The exception to that rule is when performance has already proven to be an issue  If you carry this principle in to Arrays Vs  Lists  then what you get is this   Use strongly typed lists until you hit performance problems  If you hit a performance problem  make a decision as to whether dropping out to arrays will benefit your solution with performance more than it will be a detriment to your solution in terms of maintenance

User · Answer

It is rare  in reality  that you would want to use an array  Definitely use a List lt T gt  any time you want to add remove data  since resizing arrays is expensive  If you know the data is fixed length  and you want to micro-optimise for some very specific reason  after benchmarking   then an array may be useful   List lt T gt  offers a lot more functionality than an array  although LINQ evens it up a bit   and is almost always the right choice  Except for params arguments  of course   -p  As a counter - List lt T gt  is one-dimensional  where-as you have have rectangular  etc  arrays like int    or string     - but there are other ways of modelling such data  if you need  in an object model   See also    How When to abandon the use of Arrays in c  net  Arrays  What s the point    That said  I make a lot of use of arrays in my protobuf-net project  entirely for performance    it does a lot of bit-shifting  so a byte   is pretty much essential for encoding  I use a local rolling byte   buffer which I fill before sending down to the underlying stream  and v v    quicker than BufferedStream etc  it internally uses an array-based model of objects  Foo   rather than List lt Foo gt    since the size is fixed once built  and needs to be very fast    But this is definitely an exception  for general line-of-business processing  a List lt T gt  wins every time

User · Answer

It completely depends on the contexts in which the data structure is needed  For example  if you are creating items to be used by other functions or services using List is the perfect way to accomplish it   Now if you have a list of items and you just want to display them  say on a web page array is the container you need to use

User · Answer

It is rare  in reality  that you would want to use an array  Definitely use a List lt T gt  any time you want to add remove data  since resizing arrays is expensive  If you know the data is fixed length  and you want to micro-optimise for some very specific reason  after benchmarking   then an array may be useful   List lt T gt  offers a lot more functionality than an array  although LINQ evens it up a bit   and is almost always the right choice  Except for params arguments  of course   -p  As a counter - List lt T gt  is one-dimensional  where-as you have have rectangular  etc  arrays like int    or string     - but there are other ways of modelling such data  if you need  in an object model   See also    How When to abandon the use of Arrays in c  net  Arrays  What s the point    That said  I make a lot of use of arrays in my protobuf-net project  entirely for performance    it does a lot of bit-shifting  so a byte   is pretty much essential for encoding  I use a local rolling byte   buffer which I fill before sending down to the underlying stream  and v v    quicker than BufferedStream etc  it internally uses an array-based model of objects  Foo   rather than List lt Foo gt    since the size is fixed once built  and needs to be very fast    But this is definitely an exception  for general line-of-business processing  a List lt T gt  wins every time

User · Answer

Most of the times  using a List would suffice  A List uses an internal array to handle its data  and automatically resizes the array when adding more elements to the List than its current capacity  which makes it more easy to use than an array  where you need to know the capacity beforehand   See http   msdn microsoft com en-us library ms379570 v vs 80  aspx datastructures20 1 topic5 for more information about Lists in C  or just decompile System Collections Generic List lt T gt    If you need multidimensional data  for example using a matrix or in graphics programming   you would probably go with an array instead   As always  if memory or performance is an issue  measure it  Otherwise you could be making false assumptions about the code

User · Answer

Most of the times  using a List would suffice  A List uses an internal array to handle its data  and automatically resizes the array when adding more elements to the List than its current capacity  which makes it more easy to use than an array  where you need to know the capacity beforehand   See http   msdn microsoft com en-us library ms379570 v vs 80  aspx datastructures20 1 topic5 for more information about Lists in C  or just decompile System Collections Generic List lt T gt    If you need multidimensional data  for example using a matrix or in graphics programming   you would probably go with an array instead   As always  if memory or performance is an issue  measure it  Otherwise you could be making false assumptions about the code

User · Answer

Really just answering to add a link which I m surprised hasn t been mentioned yet  Eric s Lippert s blog entry on  Arrays considered somewhat harmful    You can judge from the title that it s suggesting using collections wherever practical - but as Marc rightly points out  there are plenty of places where an array really is the only practical solution

User · Answer

Notwithstanding the other answers recommending List lt T gt   you ll want to use arrays when handling    image bitmap data other low-level data-structures  i e  network protocols

User · Answer

Unless you are really concerned with performance  and by that I mean   Why are you using  Net instead of C     you should stick with List lt    It s easier to maintain and does all the dirty work of resizing an array behind the scenes for you   If necessary  List lt   is pretty smart about choosing array sizes so it doesn t need to usually

User · Answer

Rather than going through a comparison of the features of each data type, I think the most pragmatic answer is "the differences probably aren't that important for what you need to accomplish, especially since they both implement IEnumerable, so follow popular convention and use a List until you have a reason not to, at which point you probably will have your reason for using an array over a List."

Most of the time in managed code you're going to want to favor collections being as easy to work with as possible over worrying about micro-optimizations.

User · Answer

Arrays should be used in preference to List when the immutability of the collection itself is part of the contract between the client  amp  provider code  not necessarily immutability of the items within the collection  AND when IEnumerable is not suitable   For example   var str    This is a string   var strChars   str ToCharArray        returns array   It is clear that modification of  strChars  will not mutate the original  str  object  irrespective implementation-level knowledge of  str  s underlying type   But suppose that  var str    This is a string   var strChars   str ToCharList        returns List lt char gt  strChars Insert 0   X      In this case  it s not clear from that code-snippet alone if the insert method will or will not mutate the original  str  object  It requires implementation level knowledge of String to make that determination  which breaks Design by Contract approach  In the case of String  it s not a big deal  but it can be a big deal in almost every other case  Setting the List to read-only does help but results in run-time errors  not compile-time

User · Answer

Arrays Vs  Lists is a classic maintainability vs  performance problem  The rule of thumb that nearly all developers follow is that you should shoot for both  but when they come in to conflict  choose maintainability over performance  The exception to that rule is when performance has already proven to be an issue  If you carry this principle in to Arrays Vs  Lists  then what you get is this   Use strongly typed lists until you hit performance problems  If you hit a performance problem  make a decision as to whether dropping out to arrays will benefit your solution with performance more than it will be a detriment to your solution in terms of maintenance

User · Answer

Arrays should be used in preference to List when the immutability of the collection itself is part of the contract between the client  amp  provider code  not necessarily immutability of the items within the collection  AND when IEnumerable is not suitable   For example   var str    This is a string   var strChars   str ToCharArray        returns array   It is clear that modification of  strChars  will not mutate the original  str  object  irrespective implementation-level knowledge of  str  s underlying type   But suppose that  var str    This is a string   var strChars   str ToCharList        returns List lt char gt  strChars Insert 0   X      In this case  it s not clear from that code-snippet alone if the insert method will or will not mutate the original  str  object  It requires implementation level knowledge of String to make that determination  which breaks Design by Contract approach  In the case of String  it s not a big deal  but it can be a big deal in almost every other case  Setting the List to read-only does help but results in run-time errors  not compile-time

User · Answer

Really just answering to add a link which I m surprised hasn t been mentioned yet  Eric s Lippert s blog entry on  Arrays considered somewhat harmful    You can judge from the title that it s suggesting using collections wherever practical - but as Marc rightly points out  there are plenty of places where an array really is the only practical solution

User · Answer

If I know exactly how many elements I m going to need  say I need 5 elements and only ever 5 elements then I use an array  Otherwise I just use a List lt T

User · Answer

Since no one mention  In C   an array is a list  MyClass   and List lt MyClass gt  both implement IList lt MyClass gt    e g  void Foo IList lt int gt  foo  can be called like Foo new     1  2  3    or Foo new List lt int gt    1  2  3       So  if you are writing a method that accepts a List lt MyClass gt  as an argument  but uses only subset of features  you may want to declare as IList lt MyClass gt  instead for callers  convenience   Details    Why array implements IList  How do arrays in C  partially implement IList lt T gt

User · Answer

Notwithstanding the other answers recommending List lt T gt   you ll want to use arrays when handling    image bitmap data other low-level data-structures  i e  network protocols

User · Answer

It completely depends on the contexts in which the data structure is needed  For example  if you are creating items to be used by other functions or services using List is the perfect way to accomplish it   Now if you have a list of items and you just want to display them  say on a web page array is the container you need to use

User · Answer

It is rare  in reality  that you would want to use an array  Definitely use a List lt T gt  any time you want to add remove data  since resizing arrays is expensive  If you know the data is fixed length  and you want to micro-optimise for some very specific reason  after benchmarking   then an array may be useful   List lt T gt  offers a lot more functionality than an array  although LINQ evens it up a bit   and is almost always the right choice  Except for params arguments  of course   -p  As a counter - List lt T gt  is one-dimensional  where-as you have have rectangular  etc  arrays like int    or string     - but there are other ways of modelling such data  if you need  in an object model   See also    How When to abandon the use of Arrays in c  net  Arrays  What s the point    That said  I make a lot of use of arrays in my protobuf-net project  entirely for performance    it does a lot of bit-shifting  so a byte   is pretty much essential for encoding  I use a local rolling byte   buffer which I fill before sending down to the underlying stream  and v v    quicker than BufferedStream etc  it internally uses an array-based model of objects  Foo   rather than List lt Foo gt    since the size is fixed once built  and needs to be very fast    But this is definitely an exception  for general line-of-business processing  a List lt T gt  wins every time

User · Answer

Populating a list is easier than an array  For arrays  you need to know the exact length of data  but for lists  data size can be any  And  you can convert a list into an array   List lt URLDTO gt  urls   new List lt URLDTO gt      urls Add new URLDTO         key    wiki       url    https              urls Add new URLDTO         key    url       url    http              urls Add new URLDTO         key    dir       url    https                 convert a list into an array  URLDTO   return urls ToArray

User · Answer

Another situation not yet mentioned is when one will have a large number of items  each of which consists of a fixed bunch of related-but-independent variables stuck together  e g  the coordinates of a point  or the vertices of a 3d triangle    An array of exposed-field structures will allow the its elements to be efficiently modified  in place --something which is not possible with any other collection type   Because an array of structures holds its elements consecutively in RAM  sequential accesses to array elements can be very fast   In situations where code will need to make many sequential passes through an array  an array of structures may outperform an array or other collection of class object references by a factor of 2 1  further  the ability to update elements in place may allow an array of structures to outperform any other kind of collection of structures   Although arrays are not resizable  it is not difficult to have code store an array reference along with the number of elements that are in use  and replace the array with a larger one as required   Alternatively  one could easily write code for a type which behaved much like a List lt T gt  but exposed its backing store  thus allowing one to say either MyPoints Add nextPoint   or MyPoints Items 23  X    5    Note that the latter would not necessarily throw an exception if code tried to access beyond the end of the list  but usage would otherwise be conceptually quite similar to List lt T gt

User · Answer

Notwithstanding the other answers recommending List lt T gt   you ll want to use arrays when handling    image bitmap data other low-level data-structures  i e  network protocols

User · Answer

Notwithstanding the other answers recommending List lt T gt   you ll want to use arrays when handling    image bitmap data other low-level data-structures  i e  network protocols

User · Answer

It completely depends on the contexts in which the data structure is needed  For example  if you are creating items to be used by other functions or services using List is the perfect way to accomplish it   Now if you have a list of items and you just want to display them  say on a web page array is the container you need to use

User · Answer

Unless you are really concerned with performance  and by that I mean   Why are you using  Net instead of C     you should stick with List lt    It s easier to maintain and does all the dirty work of resizing an array behind the scenes for you   If necessary  List lt   is pretty smart about choosing array sizes so it doesn t need to usually

[.net] Array versus List<T>: When to use which?

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